The book begins with AJ saying that people get confused when he tells them the name of the school he goes to. He says that the school was named after an old lady named Ella Mentry. AJ says that it would be a lot smarter if they named the school after someone famous, like a president or a popstar. On Monday morning, AJ and his classmates go down to the all-purpose room for an assembly.

After the assembly, AJ and his classmates have another math lesson from Mr. Granite. While Mr. Granite is giving his lesson, Ms. Leakey comes into the classroom, saying that it's time for health class. She makes the kids exercise. After they exercise, Ms. Leakey talks about nutrition. She tells them that what they eat is very important. She asks them if they want to be strong like her. She says that she might be stronger than Mr. Granite. Mr. Granite is then challenged to an arm wrestle with Ms. Leakey. Ms. Leakey ends up winning, and all the girls celebrate, while the boys say Ms. Leakey cheated. Then Mr. Klutz hears all the commotion and goes in to investigate. He is then challenged to another arm wrestle with Ms. Leakey. Ms. Leakey ends up winning again. Then Officer Spence comes in, and he too is challenged to an arm wrestle with Ms. Leakey. She ends up winning again.

At lunch, AJ and his friends discuss if they could get strong if they ate more fruits and vegetables. After AJ finishes eating his sandwich, he prepares to eat his dessert. But then Ms. Leakey comes in and tries to keep AJ from eating his dessert. She tells AJ what kind of chemicals are in it. AJ ponders whether or not to eat his dessert. He decides he would, because he isn't taking any chances. But Ms. Leakey won't let him. She then tells the whole cafeterium how many pounds of junk food people eat every day. Then she grabs one of the garbage cans that the kids clean their trays into and takes all the kids' desserts and throws them into the garbage can. Some kids decide to throw their desserts into the garbage can just to be safe. Then she makes all the kids follow her to the corner of the playground. She throws all the desserts into the dumpster.

After lunch, AJ has to go down to Ms. Leakey's office. He finds Ms. Leakey breathing pure oxygen in her hyperbaric chamber. She tells AJ that she rented it. She then shows AJ the stuff in her office: she has a farm in her office because she grows her own organic food, and an "endless pool" that she swims laps in. She offers AJ a bag of trail mix, but AJ doesn't want any. Ms. Leakey tells AJ that she's concerned about his diet. AJ asks why she has a punching bag in her office. Ms. Leakey tells them that when people eat junk food, it puts them in a bad mood, so it helps if they use a punching bag. She then allows AJ to try the punching bag. AJ has a lot of fun punching the bag, when suddenly, some treats come out of the bag. But Ms. Leakey won't let AJ eat the treats.

When AJ returns to his classroom, Ms. Leakey tells him that if she can try to get him to eat healthy, then the same process will follow with the other students. Then she leaves. On Friday, when AJ and his classmates get to science class, Mr. Docker introduces them to Mr. Harrison, the school's new technician. He and Mr. Docker have built a robot named Mr. Slug. They have built the robot to show what happens to people who smoke and drink and eat poorly. Mr. Docker says that Mr. Slug runs on potatoes. The kids watch as Mr. Slug stumbles across the room.

When AJ comes home from school, his mom finds a cupon for a restaurant called McLeakey's. AJ then realizes that Ms. Leakey opened her own restaurant. When AJ and his family go to the restaurant, AJ orders, but realizes that everything on the menu is healthy. AJ's dad decides they would just grab two apples from the restaurant.

The next day, AJ and his mom go shopping. AJ's mom promises AJ that she'll buy a treat for him. Before AJ and his mom finish shopping, AJ goes to get his treat. he decides on a candy bar, but Ms. Leaky jumps out of the candy box and tells AJ what they put in the candy bar. AJ then decides that he doesn't want a treat after all. That night, AJ can't stop thinking about his urge for junk food. Then he remembers that he has a leftover chocolate bar in his closet. He grabs it out of his closet. He's just about to eat it when Ms. Leakey arrives and tells AJ that people eat twenty-two pounds of candy every year. Then she takes the chocolate bar and leaves.

Now, thanks to Ms. Leakey, Ella Mentry Elementary School is getting more healthy. They have taken the soda machines out and no more candy is allowed. Ms. LaGrange is serving healthy meals, and the parents are no longer allowed to bring cupcakes on kids' birthdays and have to prepare healthy lunches for their children. AJ and his friends tell each other that Ms. Leakey might be trying to make the whole world healthy. They suspect that she might not be their real health teacher. AJ and his friends are all sad that they can't have candy anymore.

But then AJ decides to get candy from the dumpster at the corner of the playground. The kids go to the corner of the playground and look in the dumpster. They find Ms. Leakey in the dumpster eating the kids' junk food. They ask her why she's doing so, and Ms. Leakey tells a few fibs before finally accepting the fact that she's not a health teacher and that she wants everyone's junk food for herself. Then she runs away. Then words spreads and Mr. Klutz decides to fire her. He decides that it's okay to have junk food once in awhile, as a treat or for dessert, but the kids need to make sure not to overdo it. Then the kids are surprised that the new health teacher is Mr. Docker and Mr. Harrison's robot, Mr. Slug. But Mr. Slug likes being lazy and lounging around rather than getting fit. Then AJ thinks of the things that happened previously in the book, and wonders what might happen next.